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Engine Smooth Only with Carb Heat


JLang

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So flying from Michigan to Chicago area today, starting descent from cruising altitude, I reduced throttle from 5200 to about 4200, and also pulled carb heat, as I usually do.  After descending, pushed carb heat back in with throttle, and instantly the engine got very rough.  Only pulling carb heat out fully restored smooth operation.  After trying several times, I filly left carb heat pulled out and landed that way.  When taxiing, pushing carb heat in had no effect; still ran smooth.  Before taking off several hours later, checked the function of the carb heat flap and looked for any obstructions in the intake and didn't see anything.  Warmup and extended runup were fine, with only normal rpm drop and smooth operation.  Return flight was uneventful, again until descending from altitude.  This time I did not pull carb heat.  However, when I pulled throttle, it again got very rough.  When increasing throttle again after descent it got even rougher, but when I pulled carb heat it instantly smoothed out.  Again I decided to land that way, and when pushing carb heat in during taxi, there was no effect.

 

This does not sound like carb ice to me, which from what I learned makes the engine run rough when carb heat is pulled, until the ice is gone.  This was the opposite.  I did recently get the plane back from annual, including carb balance, though I have had several flights since then without experiencing this.

 

Thoughts?

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Thanks for the responses.  '07 SW.  I did check the choke, and it was fully forward.  I believe the only related maintenance during the recent annual was the carb sync,  I will check the hoses; I did not see anything during my quick inspection after the first occurrence but did not specifically look for this.

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12 minutes ago, Tom Baker said:

One other thing you might check. Remove the 3" ducts from the rear of the carbs and reach in and feel if the slide is moving up and down smoothly. I know of a couple cases where the slide had a residue on it that was causing it to stick. 

Excellent tip.

Thanks Tom.

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3 hours ago, JLang said:

Thanks for the responses.  '07 SW.  I did check the choke, and it was fully forward.

The lever may be forward, but did you confirm the choke pivot on each carb was moving all the way to the stops in both the on and off positions?  The lever can be off and you could still have partial choke on one or both carbs.

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Application of carb heat causes intake air flow to be hotter (i.e., thinner) and typically results in a richer % mixture.  If your engine is running roughly due to its being a bit on the lean side during reduced-power descent, application of carb heat may enrichen the mixture % sufficiently to smooth out the engine.  Perhaps your engine is running a bit lean due to debris in the carbs or a mis-set venturi needle??  Just a thought.

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9 hours ago, DHeal said:

Application of carb heat causes intake air flow to be hotter (i.e., thinner) and typically results in a richer % mixture.  If your engine is running roughly due to its being a bit on the lean side during reduced-power descent, application of carb heat may enrichen the mixture % sufficiently to smooth out the engine.  Perhaps your engine is running a bit lean due to debris in the carbs or a mis-set venturi needle??  Just a thought.

I don't really think the carb heat on the CTSW is hot enough to make much of a mixture change.

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So I found the culprit: one hose from the carb to the airbox was disconnected. Re-ziptied. I am a bit skeptical it will hold due to the small overlap. Is there a trick to better secure?

Again, thanks to all for the suggestions.

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Are you talking about the carb vent tube to the Rotax air box? If so just use a small wire tie or some safety wire to secure it. If this issue does come off it cause a carb imbalance and rough running. It is usually at higher rpms and not down in the idle range.

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